1. Field of the Invention
The present invention relates to the oil and gas sector and in particular to a proppant for use in methods of stimulating oil or gas reservoirs by hydraulic fracturing.
2. Description of the Prior Art
The principle of hydraulic fracturing is to create fractures in a reservoir in the wellbore region to increase permeability near the wellbore and improve oil production. Specific fluids properties and pumping pressure properties are used to create these fractures. However, once pumping ends, the pressure reduces, or the fluid used for hydraulic fracturing flows back or is removed, the walls of these fractures tend to close due to reservoir pressure. To prevent this closure, it is common to add a granular material referred to as proppant to the fracturing fluid. The proppant is delivered into the fracture along with the fracturing fluid. It is desirable that the proppant remain in the fracture to mechanically prevent closure of the reservoir after pressure is reduced. The proppant therefore has to have a specific set of properties, such as suitable strength, to withstand prolonged loading, and porosity, and must be of a particular size. It must be strong enough to resist reservoir pressure and light enough to be delivered into the fracture before it settles. Thus, while most materials possess high durability and high density, the optimal proppant should possess high strength and low density.
At present a thrust in the development of proppant technology and chemical composition is focused on lowering the density of the proppant and certain trends can be identified in the development of new materials which combine high strength and low density. One area is development of low-density materials for the manufacture of proppant: For example, one proposal is to use polymeric granules, in particular, granules based on polystyrene divinylbenzene. Low-density bauxites with high durability properties have also been developed. Another area is use of composite materials. One proposal described in U.S. Pat. No. 4,923,714 is the use of ceramic proppants with a coating made of a thin polymeric film that prevents the penetration of fluids into the proppant, reducing the effective density of the proppant. Another as described in U.S. Pat. No. 6,632,527 makes use of a composite material based on a mixture of ceramic particles and polymer resin. A further proposal described in U.S. Pat. No. 6,528,157 relates to particles with a coating fabricated from resin and fibrous material. A hollow proppant is described in U.S. Pat. No. 4,547,468. All of the preceding references are hereby incorporated by reference.
A method is known of delivering proppant into a fracture based on a reduction in the settling rate. The effect is achieved in several ways. One method is by increasing the viscosity and density of the fluid used for hydraulic fracturing. However particular fluid densities and viscosities are required to be useful as a fracturing fluid. Any large change in these properties is therefore difficult to achieve. Another method is by reducing the density of the proppant. In the majority of cases however reduction in the density of the proppant leads to a decrease in its strength. The use of special materials often leads to significant cost increases. Another method is by introducing special additives into the fluid used for hydraulic fracture, for example polymeric fibers, to reduce the proppant settlement rate by mechanically restricting settlement of the proppant.
Use of these described methods however generally leads to cost increases. The benefits of achieving the optimum relationship of high strength and low density for the proppant may not compensate or offset for the additional expenses and the complexities of production. An aim of the present invention is the creation of a proppant having desirable physical properties that provide improvements in economy and effectiveness in use.